Ionizing radiation—such as, for example, the radiation generated by radioactive materials, or the radiation present in planetary radiation belts—is known to cause substantial problems with electronic circuits and systems. Even relatively mild amounts of radiation cause errors in semiconductor memories. Larger amounts of radiation might even cause permanent damage to electronic devices.
RFID tags are becoming ubiquitous and useful in a variety of applications. However, in applications where exposure to ionizing radiation might occur, RFID tags suffer from the same problems as other electronic systems. This is true for all types of RFID tags. Commonly-used types of RFID technology comprise, for example, passive RFID tags without any internal source of power; semipassive RFID tags which comprise an internal battery for powering electronic circuitry and, possibly a radio receiver, but might not comprise an active radio transmitter; and active tags, which usually comprise a radio receiver and a radio transmitter, and an independent source of power such as a battery.
Passive and semipassive RFID tags make use of so-called passive-radio technology. Passive-radio technology is a particular type of radio technology wherein a radio circuit generates a radio-frequency signal from another radio-frequency signal without active amplification. In particular, for example, a passive-radio radio circuit might generate a reflection of an incoming radio-frequency signal while imparting to it a modulation based on another signal that is not a radio-frequency signal.
It is well known in the art how to make electronic systems that can tolerate ionizing radiation. Such electronic systems are referred to as “radiation-hardened”. Techniques for making radiation-hardened electronic systems generally involve using electronic devices that are different from those used in standard electronic systems. For example, integrated circuits for radiation-hardened systems are fundamentally different from integrated circuits used in consumer electronics, and they can tolerate much larger amounts of ionizing radiation. Various techniques are known in the art for making such radiation-tolerant integrated circuits and other radiation-tolerant electronic devices for radiation-hardened electronic systems.
Radiation-hardened electronic systems that use radiation-tolerant electronic devices are much more expensive than equivalent systems made with general-purpose commercial devices because the radiation-tolerant devices are intrinsically more expensive and because they do not benefit from the same economies of scale as general-purpose commercial devices.
Another technique for making radiation-hardened electronic systems is to use shields. Depending on the type of ionizing radiation present, shielding can be very effective. For example, x-rays used for medical applications are very effectively blocked by metal sheets. Even relatively thin metal sheets can be sufficient to substantially attenuate such X-rays. Therefore, for an electronic system that must operate in the presence of such X-rays, for example, enclosing it in a metal sheet is an effective way to achieve some level of radiation hardening. In many applications, shielding with metal sheets can be sufficient to enable an electronic system to be made with general-purpose commercial devices instead of special-purpose radiation-tolerant devices.
Metal sheets, unfortunately, are effective for blocking not only ionizing radiation, but also radio signals. Therefore, an electronic system that comprises a radio transmitter or receiver cannot be enclosed in metal sheets because such sheets would also block the radio signal and prevent the desired functionality. This is particularly unfortunate for RFID tags, where low cost is frequently an important objective. Shielding with metal sheets is, generally, a much less expensive way of achieving radiation hardening, compared to using radiation-tolerant devices.
Another reason why it's difficult to make RFID tags with radiation-tolerant devices has to do with power consumption. Generally, radiation-tolerant electronic devices are not as power efficient as non-radiation-tolerant electronic devices. In the case of RFID tags, low power consumption is an important and difficult-to-achieve feature in general. If radiation-tolerant devices must be used, it becomes even more difficult to achieve low power consumption.
FIG. 1 shows a layout of an RFID tag 100 in accordance with the prior art. RFID tag 100 comprises metal loop 110, and radio circuit 120, arranged as shown. Typically, metal loop 110 is made out of a thin layer of metal deposited on an insulating substrate (not shown in FIG. 1) that also provides mechanical support for radio circuit 120. In RFID tag 100, metal loop 110 acts as an antenna for radio circuit 120.
If an RFID tag such as RFID tag 100 is used in an environment where ionizing radiation in present (for example, in a spacecraft) the ionizing radiation that strikes radio circuit 120 might impair its operation.
FIG. 2 shows a layout of an RFID tag 200 in accordance with the prior art. RFID tag 200 comprises metal pattern 210, and radio circuit 220, arranged as shown. Typically, metal pattern 210 is made out of a thin layer of metal deposited on an insulating substrate (not shown in FIG. 2) that also provides mechanical support for radio circuit 220. In RFID tag 200, metal pattern 210 acts as an antenna for radio circuit 220.
If an RFID tag such as RFID tag 200 is used in an environment where ionizing radiation in present (for example, in a spacecraft) the ionizing radiation that strikes radio circuit 220 might impair its operation.